A number of pulse width modulation (PWM) inverters have been developed for converting direct current (DC) power or fixed frequency alternating current (AC) power to variable frequency AC power for the purpose of driving AC synchronous or induction motors at variable speeds. Also, a large number of permanent magnet (PM) motors have been developed which resemble AC synchronous motors in construction but are typically driven by a controller that only provides an electronically commutated waveform to the stator windings, similar to that generated by the mechanical commutator in a DC motor. However, full-fledged variable frequency inverters are also being used more commonly in recent years to drive these PM motors, particularly in the higher rating devices. This is primarily because the high frequency components generated by hard-switching electronic commutation results in unacceptable dielectric cycling of insulation as well as hysteresis and eddy current heating in larger motors unless expensive insulation, lamination and conductor types are used in the construction of the motor.
The use of PWM inverters allows effective operation of higher rating motors incorporating conventional lamination and conductor types. However, these motor controllers are still typically limited to ratings of several hundred kilowatts. This rating limitation is primarily due to the ratings of power switching devices currently available and the circuit configurations in which they are typically used. Currently available power switching devices for high voltage (e.g., 500-4500v), high speed switching (e.g., 10-40 KHZ), and high current (e.g., 50-400 A) applications are limited to insulated-gate bipolar transistors (IGBT's). Certain other devices can be used, such as thyristors, plain bipolar junction transistors (BJT's), Darlington BJT's and metal oxide semiconductor (MOS) controlled BJT's with some associated compromise in performance or cost. Other devices under development, such as MOS controlled thyristors (MCT's), promise higher current capacity with comparable voltage and switching speed ratings, which would make current PWM inverter configurations practical at power levels in the several megawatt range or higher. Unfortunately, these devices will require special protection features to avoid the hazards of high-current explosive faults and may require higher switching speeds than devices of lower ratings to avoid parasitic losses in the stator windings and laminations due to harmonic distortion of the inverted waveform. This latter problem could lead to expensive modifications in the motor construction (e.g., thinner laminations and smaller wire gauges), particularly in large motors, to avoid damage to insulation from excessive heating and unacceptable performance losses.